For example, in the field of railway signalling, a detection of train is performed by using track circuits. However, as a secondary function, these track circuits are provided with rail failure detecting functions for detecting whether there is a break in the rails.
Incidentally, in recent years, train detecting systems using radio transmission for the purpose of facilitation of safety have been considered. In the case where a train detecting system using radio transmission is adopted, since this system does not have a function for detecting rail breaks, it is necessary to provide separately a facility for detecting rail failures such as breaks and the like. An advantage of a train detecting system using radio transmission is that signal line cabling is not required. Accordingly, it is desirable for a rail failure detecting apparatus used together with a train detecting system using radio transmission to be one that does not require the signal line cabling.
For such a rail failure detecting system, systems using ultrasonic waves are well known (refer to International Publication WO98/7610 and U.S. Pat. 5,743,495).
FIG. 1 shows an example of a conventional break detecting system for detecting rail breaks over a long distance using ultrasonic waves.
In FIG. 1, a large number of terminal devices 21, 22, 23, . . . are installed at intervals along a rail 1. The terminal devices 21, 22, 23, . . . are provided with ultrasonic wave transmitter-receivers 31, 32, 33, . . . and communication units 41, 42, 43, . . . respectively, and are connected to a central processing unit 5 via a communication line 6 so as to be able to communicate with the central processing unit 5.
In this conventional system, ultrasonic waves are transmitted from each terminal device to the next terminal device such that the terminal device 21 transmits ultrasonic waves to the next terminal device 22, the terminal device 22 transmits ultrasonic waves to the next terminal device 23, and the terminal device 23 transmits ultrasonic waves to the next terminal device. Each of the terminal devices 21, 22, 23, . . . periodically informs the central processing unit 5, through each of the communication units 41, 42, 43, . . . , whether ultrasonic waves have been transmitted or received.
For example, if a rail break occurs between the terminal device 21 and the terminal device 22, an ultrasonic wave signal transmitted from the terminal device 21 is not received by the terminal device 22. If there is no information from the terminal device 22 to the central processing unit 5 that the signal has been received, then the central processing unit 5 judges that there is a break in the rail between the terminal device 2, and the terminal device 22.
Furthermore, in the case where the location of a rail break is detected by the system in FIG. 1, when each of the terminal devices 21, 22, 23, . . . receives reflected waves, it immediately informs the central processing unit 5 that reflected waves have been received. The central processing unit 5 can calculate the location where reflection occurred, that is, the location of a break in the rail, based on the time the ultrasonic waves were transmitted and the time the reflected waves were received.
Incidentally, the transmission distance of ultrasonic waves depends on the form and installation condition of an ultrasonic transmission medium. For example, in the case of a pipe, equipments for fitting pipe increase the attenuation of ultrasonic waves. Furthermore, in the case of a rail, the attenuation of ultrasonic waves becomes large by sleeper and rail fastenings. Generally, the detection range in the case of pipe failure detection using reflection of ultrasonic waves is approximately few score meters, and the detection range in the case of rail break detection is from 1 to 2 km.
Therefore, in the case of detecting breaks in a rail or pipe installed over a long distance, in the conventional system in FIG. 1, if the number of terminal devices connected to the central processing unit via the communication line is increased, there is a problem in that control of the network comprising terminal devices, communication lines and a central processing unit becomes complicated.
A method has also been reported for performing long distance transmission by generating powerful ultrasonic waves. However, since there are possibilities of the ultrasonic wave generating apparatus becoming too big, and the ultrasonic transmission medium itself getting damaged, it is difficult to utilize such a method for detecting breaks in pipes and rails.
The present invention takes such conventional problems into consideration, and has an object of providing a failure detecting system capable of reducing the number of terminal devices connected to a communication line of a network, and also simplifying the network.